When taking Raman spectra of gases or other nominally transparent fluids it is desirable to generate as much signal as possible. Several techniques have been developed to increase these signal levels. One approach, described in U.S. Pat. No. 5,956,138, relies on multipass techniques that produce a multiplicity of focal images. Another approach, disclosed in U.S. Pat. No. 7,692,786 uses retro reflections. Each copy of the focal image adds to the overall signal level and controls signal level.
Certain proposed Raman systems use ellipsoidal reflectors, since rays passing through one point are directed to the other. However, the ellipsoidal reflectors in existing systems are typically used for beam redirection as opposed to signal amplification. For example, U.S. Pat. No. 3,704,951 uses mirrors, which may be ellipsoidal, in such relation to a sample zone that source light is caused to pass many times through a sample in that zone for increasing substantially the intensity level of Raman light emission from the sample. However, the multiple “bounces” of the light are limited to illumination and not collection, which is derived form a separate lateral path. The emphasis is on providing a multi-pass “pencil beam” from a laser, and not magnification of numerical aperture.
Similarly, U.S. Pat. No. 3,825,325 uses a collimated pencil beam or ray of light to multi-pass at one focus of an ellipsoid for lateral collection. Numerical aperture is actually diminished. In U.S. Pat. No. 6,721,049, an ellipsoid is used as a single-reflection imager, not a multi-bounce re-image amplifier. Published U.S. Application Serial No. 2007/0010727 is directed to a catheter head having means for directing of radiation to a blood detection volume, means for receiving of return radiation from the blood detection volume, and means for transmitting of the return radiation to means for analysis of the return radiation for determination of at least one property of the blood. One embodiment comprises a spherical mirror for reflection of the laser radiation back into the blood detection volume, and an ellipsoidal mirror, wherein the blood detection volume includes one of the focal points of the ellipsoidal mirror. However, the ellipsoid is used to image a fiber, and not to trap/form re-imaging for amplification and/or low noise.